Environmental concerns have led to continued efforts to reduce the NOx emissions of compression ignited (diesel) internal combustion engines. The latest technology being used to reduce the NOx emissions of diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
Diesel fuel contains sulfur. Even “low-sulfur” diesel fuel contains 300 to 400 ppm of sulfur. When the fuel is burned in the engine, this sulfur is converted to SOx. In addition, one of the major by-products of the combustion of a hydrocarbon fuel is water vapor. Therefore, the exhaust stream contains some level of NOx, SOx and water vapor. In the past, the presence of these substances has not been problematic because the exhaust gases remained extremely hot, and these components were exhausted in a disassociated, gaseous state. However, when the engine is equipped with an EGR and the exhaust gas is mixed with cooler intake air and recirculated through the engine, the water vapor can condense and react with the NOx and SOx components to form a mist of nitric and sulfuric acids in the EGR stream. This phenomenon is further exacerbated when the EGR stream is cooled before it is returned to the engine.
In the presence of these acids, it has been found that soot levels in lubricating oil compositions build rapidly, and that under said conditions, the kinematic viscosity (kv) of lubricating oil compositions increase to unacceptable levels, even in the presence of relatively small levels of soot (e.g., 3 wt. % soot). Because increased lubricant viscosity adversely affects performance, and can even cause engine failure, the use of an EGR system requires more frequent lubricant replacement. It has been found that the simple addition of dispersant does not adequately address the problem.
Therefore, it would be advantageous to identify lubricating oil compositions that better perform in diesel engines equipped with EGR systems. Surprisingly, it has been found that by selecting certain additives, specifically certain viscosity modifiers, dispersants and/or detergents, and/or controlling the level and basicity of dispersant nitrogen, the rapid increase in lubricant viscosity associated with the use of engines provided with EGR systems can be ameliorated.